An oil burner furnace has a combustion chamber which conventionally is connected through a smoke pipe to a chimney flue passage. During normal operation of the furnace hot combustion gases arising from combustion in the chamber flow out of the chamber through the smoke pipe into the chimney flue. These hot gases are slightly buoyant relative to ambient atmosphere. Thus, they rise up through the flue passage and exit from the top of the chimney. Upward flow of hot combustion gases through a chimney flue passage accompanied by flow of these hot gases through the smoke pipe is called a "flue draft". Flue draft normally exerts a slight suction action on a combustion chamber. Such suction action during normal operation of an oil burner produces a desired slightly reduced pressure, i.e., a sub-atmospheric pressure, within a combustion chamber relative to ambient pressure outside of the furnace.
This normal slightly sub-atmospheric operating pressure within a combustion chamber is called a "negative pressure". In summary, for providing good combustion conditions it is normal and desired for the combustion chamber of a conventional home heating oil burner furnace to be operating at a "negative pressure" due to an appropriate "flue draft".
There are occurrences which may interfere with, or interrupt, or impede a normal flow of hot combustion gases through a smoke pipe and up a chimney flue passage. In other words, occasions may arise when a flue draft becomes reduced or blocked. A reduced or blocked flue draft which is sufficiently abnormal so as to cause poor combustion with resultant smoke and oily soot becoming forced out of the combustion chamber by abnormal persisting positive pressure so as to enter into living space in a building is called a flue draft "malfunction". Such malfunction can be caused by a variety of adverse factors, such as: a clogged chimney flue passage, a severely rusted perforated smoke pipe, a broken open connection in a smoke pipe, a smoke pipe falling detached from a furnace outlet or detached from a chimney, or a draft-regulator valve disc falling from its pivot, thereby leaving a wide-open smoke-pipe Tee, etc.
A clogged chimney flue passage can result from deterioration of a flue tile such that broken tile pieces fall down from time to time within the flue passage. These accumulating tile pieces can pile up within a flue passage so as to impede or block the flow of hot combustion gases from the smoke pipe into the flue passage. The occupants of a house may be away on vacation during part of a winter and may not be aware of a flue draft malfunction due to deterioration or clogging of a smoke pipe or flue. Or the occupants may have a very busy schedule and not have an opportunity to notice malfunction of a flue draft caused by occurrences of an adverse factor or factors.
As will be explained, problems can arise from malfunction of a flue draft. Interference with, or interruption of, or impedance preventing normal flow of hot combustion gases through the smoke pipe and up the chimney flue passage prevents establishment of a normal flue draft suction action, thereby causing loss of desired negative pressure in the combustion chamber. When malfunction of a flue draft has occurred the usual negative pressure is replaced by an abnormal positive pressure. This abnormal positive pressure results from the fact that an oil burner blower pumps air into the combustion chamber. During flue draft malfunction the blower creates a positive pressure, sometimes called a "back pressure", in the combustion chamber.
This abnormal positive pressure in the combustion chamber causes hot smoky and oily sooty combustion gases to seek exits from the chamber through every available opening and crack. One such exit is through an inspection opening (peep hole) in a furnace inspection door.
As noted above, combustion under conditions of abnormal positive pressure (back pressure) becomes very smoky and sooty. Oily soot and smoke exiting from a positively pressurized combustion chamber can flood throughout a house resulting in serious oily smoke/soot damage. Moreover, in addition to creation of smoke/soot damage, the poor combustion produces excessive amounts of carbon monoxide which may escape from the combustion chamber along with the smoke and soot. Also, build-up of oily soot in and around a furnace, its smoke pipe and flue can become a potential fire hazard.
A conventional optical-type oil burner safety control will not shut off an oil burner when there is malfunction of a flue draft which is causing positive (back) pressure and smoky, sooty combustion in a combustion chamber. Conventionally, an optical sensor is utilized for sensing light from a combustion flame. If a flame fails to ignite within a predetermined time interval after an oil burner is turned on, for example an interval of about 40 to about 45 seconds, then the optical-type control will de-energize the oil burner to turn it off. However, if a flame commences within the predetermined time interval, a conventional optical-type control will allow the burner to continue firing regardless of whether the flame is sooty or normal. Consequently, an optical sensor will allow a badly sooting combustion condition to continue uninterrupted so long as a house thermostat or other temperature sensor is calling for heat to be provided by the oil-fired furnace.